Catalytically Active μ-Oxodiiron(IV) Oxidants from Iron(III) and Dioxygen

Abstract

The reaction between an Fe(III) complex and O(2) to afford a stable catalytically active diiron(IV)-mu-oxo compound is described. Phosphonium salts of orange five-coordinated Fe(III)-TAML complexes with an axial aqua ligand ([PPh(4)]1-H(2)O, tetraamidato macrocyclic Fe(III) species derived from 3,3,6,6,9,9-hexamethyl-3,4,8,9-tetrahydro-1H-1,4,8,11-benzotetraazacyclotridecine-2,5,7,10(6H,11H)-tetraone) react rapidly with O(2) in CH(2)Cl(2) or other weakly coordinating solvents to produce black mu-oxo-bridged diiron(IV) complexes, 2, in high yields. Complexes 2 have been characterized by X-ray crystallography (2 cases), microanalytical data, mass spectrometry, UV/Vis, Mossbauer, and (1)H NMR spectroscopies. Mossbauer data show that the diamagnetic Fe-O-Fe unit contains antiferromagnetically coupled S = 1 Fe(IV) sites; diamagnetic (1)H NMR spectra are observed. The oxidation of PPh(3) to OPPh(3) by 2 was confirmed by UV/Vis and GC-MS. Labeling experiments with (18)O(2) and H(2)(18)O established that the bridging oxygen atom of 2 derives from O(2). Complexes 2 catalyze the selective oxidation of benzylic alcohols into the corresponding aldehydes and bleach rapidly organic dyes, such as Orange II in MeCN-H(2)O mixtures; reactivity evidence suggests that free radical autoxidation is not involved. This work highlights a promising development for the advancement of green oxidation technology, as O(2) is an abundant, clean, and inexpensive oxidizing agent.

Oxygen reduction at negatively charged iron porphyrins heat-treated and bridged by alkaline-earth metal ions

“…Thereafter, Bouwkamp-Wijnoltz et al [18] verified the presence of all iron One of the authors [5] has suggested, based on spectroelectrochemical experiments, that oxygen reduction at FeTPPS 4 in a polypyrrole matrix in acidic solutions proceeds with Fe(III) but not with Fe(II). Recently, a study reported that oxygen does not coordinate Fe(II) but rather Fe(III) to form an oxo dimer of Fe(IV) [9]. The iron complex evaluated in the aforementioned study was not a porphyrin, but did contain a N 4 chelation site.…”

“…A new oxygen reduction mechanism has been proposed in the face-to-face cobalt porphyrin [8], in which the formation of a !-cation radical is a key factor in the coordination of oxygen to the metal site. Recently, a study reported that oxygen coordinates Fe(III) rather than Fe(II) to form an oxo dimer of Fe(IV) [9], although this study was conducted with a non-heme complex. The results cited above indicate that the formation of a high-valent metal may be important in oxygen reduction at metallomacrocycles.…”

A synthetic precedent for the [Fe IV 2 (μ-O) 2 ] diamond core proposed for methane monooxygenase intermediate Q

“…In 2001, we claimed to have generated such a complex (15); however, additional data obtained in our laboratories strongly suggested that the compound should be reformulated as an Fe IV (OH)(OOR) species (16). In 2005, Collins and coworkers (17) reported the crystallographic and spectroscopic characterization of the first example of a ( -oxo)diiron(IV) complex; in this case, the high oxidation state was stabilized by the tetraanionic TAML ligand. Here we report a diiron(IV) complex with an [Fe 2 ( -O) 2 ] diamond core, obtained by one-electron oxidation of its [Fe III Fe IV ( -O) 2 ] precursor by using bulk electrolysis.…”

Synthetic iron-oxo “diamond core” mimics structure of key intermediate in methane monooxygenase catalytic cycle

“…Lastly, given the tremendous oxidation power of MMO-Q, which is Nature's only species known to date that is capable of converting methane to methanol, synthetic complexes possessing an [Fe IV 2 ( -O) 2 ] diamond core may also represent promising candidates for the advancement of green oxidation technology. Indeed, research carried out by Collins and coworkers (16)(17)(18) over the last two decades has revealed that high-valent ironoxo species are well suited for catalyzing a wide range of technologically important oxidations, such as the rapid bleaching of water-soluble dyes, the decolorization of pulp mill effluents, and the complete remediation of chlorophenol persistent pollutants.…”